1. Field of the Invention
The present invention relates to the treatment/-purification of aqueous media utilizing a particular coagulant, and, more especially, to such treatment utilizing a combination coagulant comprising an aluminum polymer and a ferric salt.
2. Description of the Prior Art
It is known to this art that the principal installations intended for the treatment of aqueous media (surface waters, effluents, residual waters, waste waters, etc.) essentially consist, on the one hand, of a gravity clarifier based on the principle of sedimentation, i.e., the elimination of solids in suspension in the water by gravity settling, and, on the other hand, by filtering means.
The settling basin thus contains, at the base thereof, a bed of sludge, and floating on the surface is a clarified liquid effluent which is removed by overflow.
It has also long been thought that the use of various coagulating agents capable of improving the conditions of sedimentation would enhance purification, and thus the quality of the waters treated.
Exemplary such coagulants include ferrous chloride, ferric chloride, ferrous sulfate, ferric sulfate, and ferric chlorosulfate.
Basic aluminum sulfates and, in particular, basic aluminum chlorosulfates, are also representative.
Most of these coagulants are indeed effective. Nevertheless, in recent times a need has arisen for a process providing lower and lower residual amounts of aluminum and organic materials in the waters treated, such as to satisfy increasingly severe hygienic and environmental standards. Need also exists for processes which are as flexible as possible, i.e., processes in which the ultimate quality of the treated water would be less dependent on the operation and construction of installations of this type.
For this purpose, the use of mixtures of iron and aluminum salts has been studied, for example by Johnson and Amirtharajah, featuring a mixture of ferric chloride and aluminum sulfate. Unfortunately, their studies lead to the conclusion that the use of such mixtures does not result in improved coagulation (see the Johnson and Amirtharajah article "Ferric Chloride And Alum As Single And Dual Coagulants", published in the May, 1984 issue of the Journal of the American Water Works Association.